Chemical vapor deposition devices have been used in the deposition of thin films on semiconductor substrates. In such a device, a gas containing the structural elements of the film material to be formed is first fed into a chamber, followed by heating the gas to induce a chemical reaction to deposit the desired film on a semiconductor substrate.
In a typical single-wafer CVD device, the platform that is used to hold the semiconductor substrate in place for the deposition process is called a susceptor. The susceptor is usually constructed of a thin plate for low mass and a surrounding rim for rigidity. At this time, the diameter of a susceptor in a typical reaction vessel is approximately 9" (228 mm) while the diameters of wafers being coated are normally 6" or 8" (152 or 203 mm). The susceptor can be made of graphite or aluminum coated with a silicon carbide coating so that it can be heated to the deposition temperature of the thin film without significant distortion. Still others have proposed methods of treating metal surfaces to improve their durability at high temperatures. For instance, U.S. Pat. No. 5,201,990 to Chang et al, assigned to the common assignee of the present invention, discloses a process for treating aluminum surfaces in a vacuum apparatus with a plasma consisting of a nitrogen-containing gas of either nitrogen or ammonia.
To select a suitable material for the susceptor used in a CVD device, the following criteria must be met. First, the thermal conductivity of the material must be sufficiently high such that any distortion in its dimensions and any deterioration in the material at the high operating temperatures of the CVD device is minimized. Secondly, when in-situ plasma chamber cleaning methods are utilized, the material used for the susceptor must be resistant to any corrosive effect of the plasma. Thirdly, the susceptor material must be of high purity in order to preclude contamination of the semiconductor substrait by impurities contained in the material.
As a consequence, metallic materials that are electrically conductive and that have high thermal conductivity are conventionally used for the susceptor. In particular, certain metal alloys of nickel such as Nonel.TM., Hastelloy.TM., etc., are frequently used for their resistance to fluorine-containing plasma which is one of the highly reactive plasmas often used in in-situ plasma chamber cleaning steps. Ceramic materials such as silicon carbide and graphite have also been used. In devices where the susceptor plate has to act as a high-frequency electrode, a conductive material such as metal must be used,
When conventional materials such as Monel.TM., Hastelloy.TM., silicon carbide and graphite are used as the susceptor material, a protective coating layer is normally required to cover the surface of the susceptor and to protect it from the corrosive effect of the plasma.
When metallic materials are used in the susceptor, plastic deformations in the material occur in response to sudden changes in temperature. This leads to the problem of the protective film peeling off due to the difference in the values of coefficient of thermal expansion between the protective film and the metal material.
When ceramic materials are used in the susceptor, even though plastic deformation does not occur, the protective film still peels off the surface of the susceptor after a certain number of plasma cleaning processes. This may be because even small differences in the values of coefficients of thermal expansion between the protective film and the ceramic materials eventually have an effect after a number of process cycles. The protective layer itself may still be gradually eroded after many cleaning operations by the highly reactive plasmas and gases employed in such cleaning operations.
A susceptor made of such conventional materials therefore has problems in achieving both long term durability and reliability.
It is therefore an object of the present invention to provide a novel susceptor plate made of a suitable material that does not have the shortcomings of the prior art susceptor materials.
It is another object of the present invention to provide a novel susceptor plate made of a ceramic material that does not require a protective coating for achieving long term durability and reliability.
It is yet another object of the present invention to provide a novel susceptor plate made of an aluminum nitride material that does not require a protective coating when the susceptor plate is used in a CVD chamber.
It is a further object of the present invention to provide a novel susceptor assembly of high durability to include a susceptor plate made of an aluminum nitride material, an electrode plate installed under the susceptor plate, and an electrode cover member made of a ceramic material to enclose the electrode.